Liquid-piston type slurry pumping system

ABSTRACT

A liquid-piston type slurry pumping system is composed of a rotary pump unit for delivering a pressurized operating fluid having a specific gravity different from that of the slurry, at least two valve boxes each having a suction valve and a delivery valve connected to a slurry pipe line, at least two liquid containers connected respectively to the valve boxes, passage means extending between the containers and the pump unit, means for changing over the passage means thereby to cause the operating fluid to flow alternately into the liquid containers, detecting means provided in each of the liquid containers for detecting excessive displacement of a boundary surface formed between the operating fluid and the slurry, and a controller for controlling the operation of the fluid-passage changeover means in response to a signal from the detecting means, whereby the movement of the boundary surfaces in the containers is controlled by the controller for stabilizing the operation of the pumping system.

United States Patent 1191 Tsukamoto June 24, 1975 [75 Inventor: Junya Tsukamoto, Hyogo, Japan [73] Assignee: Mitsubishi Kinzoku Kabushiki Kaisha, Tokyo, J apan [22] Filedw Oct. 26, 1973 [21] App]. No.: 409,809

[30] Foreign Application Priority Data Nov. 2, 1972 Japan 47-126937 [52] US. Cl 417/101; 417/900 [51] Int. Cl F04f 11/00; F04b 15/02 [58] Field of Search 417/101, 102, 103,125, 417/900, 398, 401

[56] References Cited UNITED STATES PATENTS 2,419,993 5/1947 Green ct a1. 417/900 X 3,303,786 2/1967 Fanshawe 417/900 X 3.740,]66 6/1973 Seki et a1. 417/900 X FOREIGN PATENTS OR APPLICATIONS 1,556,179 10/1969 Germany 417/102 245512 11/1969 U.S.S.R 417/900 2,002,190 8/1970 Germany 417/900 Primary E.\'aminerWil1iam L. Freeh Assistant Examiner-Richard E. Gluck Attorney, Agent, or F irm Wenderoth, Lind & Ponack [57] ABSTRACT A liquid-piston type slurry pumping system is composed of a rotary pump unit for delivering a pressurized operating fluid having a specific gravity different from that of the slurry, at least two valve boxes each having a suction valve and a delivery valve connected to a slurry pipe line, at least two liquid containers connected respectively to the valve boxes, passage means extending between the containers and the pump unit, means for changing over the passage means thereby to cause the operating fluid to flow alternately into the liquid containers, detecting means provided in each of the liquid containers for detecting excessive displacement of a boundary surface formed between the operating fluid and the slurry, and a controller for controlling the operation of the fluid-passage changeover means in response to a signal from the detecting means, whereby the movement of the boundary surfaces in the containers is controlled by the controller for stabilizing the operation of the pumping system.

PATENTEI] JUN 24 I975 SHEET PATENTEDJUN 24 I975 8913 52 SHEET 3 FIG.3

LIQUID-PISTON TYPE SLURRY PUMPING r I SYSTEM BACKGROUND OF THE INVENTION This invention relates generally to liquid-piston type pumping systems to be used with liquids containing foreign materials and/or abrasive particles; such as muddy water or ore slurry, and more particularly to improvements in the pumping system.

A liquid-piston type pumping system which comprises a plunger pump and a liquid container through which the plunger pump is connected to a valve box having a suction valve and a discharge valve is known. During operation, the interior of the valve box and the lower part of the liquid container is filled with a liquid containing foreign materials and/or abrasive particles, such as muddy water or ore slurry (hereinafter simply called a slurry), while the interior of the plunger pump and the upper part of the liquid container is occupied by an operating fluid, such as oil, having a lower specific gravity than that of the liquid (or slurry) and being substantially immiscible with, insoluble in, and unreactive with, the slurry.

In the liquid container, a distinct boundary surface is formed between the slurry, occupying the lower part of the container, and the operating fluid, which is immiscible with the slurry and occupying the upper part of the container, and the boundary surface is moved up suction valve and the discharge valve in the valve box connected to a slurry pipe line, and hence the slurry is pumped at a comparatively high pressure through the pumping system.

Although the above described slurry pump system is advantageous in that the plunger pump can be operated with an operating fluid absolutely free from abrasive particles and intrusion of the slurry into the plunger pump is effectively prevented by the provision of the liquid container, the plunger pump constituting an essential component of the conventional system requires a considerable size and weight when it is desired to increase the capacity of the pumping system, and the operation thereof is not sufficiently smoothand noiseless. Furthermore, a volumetric relation must be maintained between the sizes of the valve box, liquid container,

and the displacement of the plunger pump, and a mere increase in the displacement of the plunger pump will produce a hazardous exceeding of the predetermined limitations of the boundary surface in the liquid container. In addition, the maintenance of the boundary SUMMARY OF THE INVENTION In view of the above noted difficulties in the convenprovement whereby automatic operation or a remote v control of the pumping system can be executed without 21 accompanying severe vibration or abnormal pressure rise.

Another object of the invention is to provide an improved liquidpiston type slurry pump system wherein the size and weight of the entire system can be substantially reduced, and the transportation and mounting thereof can be substantially facilitated.

Still another object of the invention is to provide an improved liquid-piston type slurry pump system wherein a rotary type pump unit can be used instead of a plunger type pump unit, whereby the delivery of the pump system can be varied without any trouble.

A further object of the invention is to provide an improved liquid-piston type slurry pump system, the es sential parts of which are available on the market, whereby the production cost of the pumping system can be substantially reduced.

The above described and other objects of the present invention can be achieved by an improved liquidpistion type slurry pump system comprising: a pump unit of a type continuously delivering a pressurized operating fluid of a specific gravity different from that of slurry to be pumped and being substantially immiscible with, insoluble in, and unreactive with the slurry; at least two valve boxes each having suction and discharge valves connected respectively to a slurry pipe line; at least two liquid container means respectively connected to the valve boxes; passage means connecting the containers and sais pump unit; means for changing over the passages extended [between the pump unit and the liquid container means thereby to introduce the operating fluid into the liquid container means alternately; detecting means provided in each of the container means for detecting any displacement in excess of a predetermined value of a boundary surface formed between the fluid and slurry coexisting in the container I means during operation of the pump system; and a controller for controlling the operation of the fluid-passage ings, wherein like parts are designated by like reference numerals and characters.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

FIG. 1 is a diagram schematically showing a liquidpiston type slurry pump system constituting a first embodiment of the present invention;

.FIG. 2 is a diagram schematically showing another example of the 'liquid-piston type slurrypumpsystem constituting a second embodiment of the present invention; and I FIG.'3 is a diagram showing a modification of the liquid container means constituting an essentialcomponent of the present invention.

DETAILED DESCRIPTION OF THE INVENTION Referring now to FIG. 1 which shows an example of the liquid-piston type slurry pump system according to the present invention, two valve boxes 1 and 1a of a similar construction are provided. The valve box 1 has a suction valve 2 and a discharge valve 3, which are" connected with a suction pipe 4 and a discharge pipe 5 branched from a main slurry pipe line 6. Likewise. the valve box la has a suction valve 20 and a discharge valve 3a, which are connected to a suction pipe 411 and a discharge pipe 5a, respectively, both being also branched from the main slurry pipe line 6.

The slurry pump system further comprises two liquid container means 8 and 8a, each having two ports, one at the bottom and the other at an upper side thereof, and the valve boxes 1 and in are connected through pipes 7 and 7a to the bottom ports of the liquid container means 8 and 8a, respectively. Thus, the interior of the two valve boxes 1 an 2 and the lower halves of the liquid containers 8 and 8a are filled with slurry X or the like introduced from the suction pipes 4 and 4a as the pump system is operated.

The upper side ports of the liquid containers 8 and 8a are further connected through pipes 9 and 9a to an electromagnetic changeover valve 10 which is in turn connected through a suction pipe 12 and a discharge pipe 13 to a pump unit ll of, for instance, a turbine type rotatable in a constant direction.

The upper half of each of the liquid containers 8 and 8a and the interior of the electromagnetic valve 10 are filled with an operating fluid Y, such as oil. which does not mix with, dissolve in, or react with, the slurry to be pumped and has a lower specific gravity than that of the slurry. Since the lower half of each of the liquid containers 8 and 8a is filled with the slurry introduced through the valve boxes 1 and la from the main slurry pipe line, distinct boundary surfaces 14 and 14a are formed between the slurry X and the operating fluid Y in thecontainers 8 and 8a.

In the upper parts of the liquid containers 8 and 80, there are, provided detectors l5 and 15a for detecting variations of the boundary surfaces 14 and 14a in the liquid containers 8 and 8a. When any one of the boundary surfaces 14 and l4a .is.el evat ed until it contacts the tip of the correspondingtdetector=15 or 1511, a controller 16 is operated, and the electromagnetic changeover valve 10 is thereby activated as will be described more fully hereinafter.

The slurry pump system of this example operates as follows. In the case where the electromagnetic changeover valve 10 is so positioned as shown in FIG. 1 that the suction pipe 12 and the discharge pipe 13 are connected with the pipes 9 and 9a, respectively, the operating fluid Y flows from the upper part of the liquid container 8 through the pipe 9, a passage of the changeover valve 10, the suction pipe 12, the rotary pump unit 11, the discharge pipe 13, another passage of the changeover valve 10, and a section of pipe 9a to the upper part of the liquid container 8a as indicated by solid line arrow marks in FIG. 1. Thus, the boundary surface 14 in the liquid container 8 is elevated, and an additional quantity of the slurry is sucked from the suction pipe 4 into the valve box 1 and also into the lower half part of the liquid container 8. At the same time, the boundary surface 14a in the liquid container 8a is lowered, so that a part of the slurry X contained in the lower part of the container 8a is pushed out through the valve box la and the pipe 7a into the discharge pipe 5a and further into the delivery side of the main slurry pipe line 6.

When the bounda'rysurface 14 in the container 8 is further elevated until it contacts the tips of the detector 15, the controller 16 is operated, and the electromagnetic changeover valve 10 is transferred to another position wherein the operating fluid delivered from the rotary type pump unit 11 is passed in the direction indicated by the arrow-headed broken lines in FIG. I, and the boundary surface 14 is now pushed down and the boundary surface 14a is elevated in the respective liquid containers 8and 8a. The slurry X contained in the lower part of the container 8 is thus driven toward the valve box 1, and a part thereof is delivered into the discharge pipe 5 and further into the main slurry pipe line At an instant when the boundary surface 14a is elevated to a level at which it touches the tip of the detector 15a, the controller 16 is again operated, and the electromagnetic valve 10 is thereby switched back to its original position wherein the operating fluid is circulated in the solid-line-arrow-marked direction shown in FIG. 1, whereby the slurry is discharged from the valve box la into the main discharge pipe line 6.

The above described operation is repeated continuously until,a desired quantity of slurry is pumped up from the suction pipes 4 and 4a.and discharged into the discharge pipes 5 and 5a in an alternate ,manner.

As will be apparent from the above description, the liquid-piston type pump system of this example comprises at least a pair of valve box-liquid container assemblies and a changeover valve. Moreover since the operating fluid is passed through the changeover valve alternately into the liquid containers under the control of the controller, operable in response to the detected be used in the pumping system, whereby numerous advantageous features as described hereinbefore can be realized.

Another example of the liquid-piston type slurry pump system is indicated in FIG. 2 wherein like parts as in the first embodiment are designated by'like reference numerals and characters. 7

This system is different from that of the first example in that a piston-cylinder device 20 is interposed between the changeover valve 10 and the liquid containers 8 and 8a. The piston-cylinder device 20 comprises first and second cylinders 23 and 23a including pistons 24 and 24a, respectively, and third and fourth cylinders 26 and 26a including pistons 27 and 27a, respectively. The pistons 24 and 27 are unitarily connected together through a rod 29, and the pistons 24a and 27a are also connected together in a unitary manner through another rod 29a. An end of a first pipe 21, the other end of which is connected to one port of the changeover valve 10 having four p'orts,is connected to the region above the piston 24 in the first cylinder 23 and also to the regionbelow the piston 24a in the second cylinder 23a. An end of a second pipe 22, the other end of which is connected to another port of the changeover valve 10, is connected to the region below the piston 24 in the first cylinder 23 and the upper side of the piston 24:: in the second' cylinder 23a.

Likewise, an end of a pipe 25, the other end of which is connected towthe liquid container 8, is connected above the piston 27 includedin the third cylinder 26 and also to the fourth cylinder 26a below the piston 27a. An endof a pipe 25a, and the other end of which is also Connected to the liquid container 8a, is connected tothe third cylinder 26, in the region below the piston 27-and tothe region above the piston 27a in the fourth'cylinder 26a.

' In the second embodiment of the invention, a reservoir 30 is further provided on the suction side of the rotary pump unit P for supplying the operating fluid Y through the pump unitP, changeover valve 10, pipes 21 and 22, and through the interior of the cylinders 23 and 23a, while another kind of operating fluid Z, which does not mix with, dissolve in, or react with, the slurry X to be pumped and has a specific gravity lower than that of the slurry, is contained in the upper parts of the of the pistons 24 and 24a is sent back through the pipe 22 and the changeover valve to the reservoir 30 on the suction side of the pump unit 11.

The downward and upward movements of the pistons M 24 and 24a, respectively, cause the pistons 27 and 27a to move downward and upward as indicated by solidline -arrow marks, and a part of the intermediate operating fluid Z contained in the spaces underside and uppe rside of the pistons 27 and 27a in the third and fourth cylinders 26 and 26a, respectively, is thereby forced into the upper part of the second liquid container 8a through the pipe 25a. Thus, the boundary surface 14a in the second liquid container 8a is forced down, and a part of the slurry X contained in the valve box la is delivered into the main slurry line 6.

On the other hand, the above-mentioned downward and upward movements of the pistons 27 and 27a, respectively, cause the suction of a part of the intermediate operating fluid Z from the upper part of the first container 8 through the pipe 25 into the spaces at the upper side and the lower side of the pistons 27 and 27a, respectively. The boundary surface 14 in the container 8 is thereby elevated. and the slurry in the suction pipe 4 is thereby sucked into the valve box 1 and toward the lower part of the container 8. When the boundary surface 14 is ultimately brought into contact with the tip of the detector 15, the controller 16 is thereupon operated, the changeover valve it) thus being operated, whereby the first operating fluid Y is passed in a direction opposite to that in the above description, as indicated by arrow-headed broken lines.

In the second embodiment of the invention, since the piston-cylinder device is provided between the changeover valve 10 and the liquid containers 8 and 8a, any possibility of undesirable components, such as abrasive particles and the like, contained in the slurry being mixed with the operating fluid Y can be completely eliminated. Furthermore, when the diameter of the first and second cylinders differs from the diameter of the third and fourth cylinders, the pressure and the delivery of the liquid-piston type slurry pump system can be changed. In addition, since the operating fluid Y is acting on the slurry through the interposition of the second operational fluid Z of the above specified character, the operational fluid Y may be air or steam instead of a liquid substance, whereby a pump unit P in the form of an air turbine or a steam turbine may also be used in such a modification.

In FIG. 3, there is illustrated still anotherembodiment of the present invention, wherein two pairs of liquid containers 18, E811, and 28 and 28a are added to the existing liquid containers 8 and 8a in FIG. I. The containers 8, l8, and 28, and 8a, 18a, and 28aare connected in series, respectively, and three kindsof operating fluids Y, Z,, and Z are used therein, so that boundary surfaces 14, 14a, 31, 31a, 32, and 32a are formed between the slurry and the-operating fluid Z and between the operating fluids Z, and Z and Z and Y. The operating fluid Z may be an oil, andltheoperating fluid Z may be, for instance, water. The'boundary surface detectors 15 and 15a are providedin the liquid containers 28 and 2841 provided in the last stage.

In this embodiment of the invention, any possibility of the slurry X mixing into the operating fluid'Y can be completely eliminated, and, furthermore, the possibility of the slurry X acting on the detectors l5 and 15a can be avoided. When water is selected as the operating fluid Z as described above, any trace of the slurry X which might have intruded. in the operating fluid Z can be easily removed by replacing the operating fluid Z since water, constituting the fluid Z is cheap.

What we claim is: g

l. A liquid-piston type slurry pumping system operable in a slurry pipe line, said systemcomprlsing: a pump unit of a type continuously delivering a pressurized operating fluid which is ofa specific gravity different from that of slurry to be pumped and is substantially immiscible with, insoluble in, and unreactive with th'e slurry; at least two valve boxes each including a suction valve and a discharge valve connected respectivelytot'he pipe line; at least a pair of liquid containers connected respectively to the vaive boxes; passage means connecting said containers and said pump unit; means for changing over said passages between the pumpiunit and the liquid containers, thereby to introduce the operating fluid into the liquid containers alternately; detecting means provided in each of the containers for detecting any displacement in excess of a predetermined value of a boundary surface formed between the fluid and the slurry coexisting in the containers; a controller for controlling the operation of the fluid-passage changeover means in response to signals from said detecting means, whereby the pulsations of the boundary surfaces in the containers are controlled in response to the detected results of the detecting means, and the operation of the pumping system is thereby stabilized; and

a piston-cylinder device provided between the liquid containers and said fluid-passage changeover means, said device being comprised of means isolating the operating fluid from the slurry but trans mitting hydraulic displacement between the operating fluid and the slurry, first and third cylinders respectively including pistons connected together operatively, and second and fourth cylinders respectively including pistons connected together operatively, the first side of the piston in the first cylinder and the second side of the piston in the second cylinder being connected to a passage in said fluid passages changing over means, the second side of the piston in the first cylinder and the first side of the piston in the second cylinder being connected to another passage in said fluid passages changing over means, the first side of the piston in the third cylinder and the second side of the piston in the fourth cylinder being connected to one liquid container, the second side of the piston in the third cylinder and the first side of the piston in the fourth cylinder being connected to the other liquid container, said operating fluid occupying both sides of each of the first and second cylinders, and the interiors of the third and fourth cylinders being occupied by an additional operating fluid which forms a boundary surface in each of the liquid containers between itself and the slurry.

2. a liquid-piston type slurry pumping system as set forth'in claim 1 wherein said operating fluid used in the pumping unit is a gas such as air or steam.

3. =ln a vliquid-piston type slurry pumping system for conducting a slurry through apipeline and including pumping means having a suction side and a pressure side and continuously delivering a pressurized operating fluid having a specific gravity lower than that of the slurry and substantially immiscible with, insoluble in. and unreactive with the slurry, at least one pair of valve boxes connected, respectively, to the pipeline in parallel relationship to each other and each including a suction valve and a discharge valve, at least one pair ofliquid containers respectively connected to said valve boxes, a boundary surface formed in each of said containers between operating fluid and slurry contained therein, passage means connecting said containers and said pumping means, means for changing over said passage means to alternatively introduce the operating fluid delivered from said pumping means into one of said containers, detector means for detecting a level of said boundary surface in each of said containers, and control means for controlling the operation of said changeover means in response to detection of the level by said detecting means, the improvements comprising:

said detector means for each container being comprised of means for detecting a displacement of said boundary surface beyond a predetermined raised level in the container, each container being connected at an upper portion thereof to said passage means;

said changover means being comprised of a single sliding changeover valve movable between a first position in which a first of said containers is connected to the suction side of said pumping means and a second container is connected to the pressure side and a second position in which said first container is connected to the pressure side of said pumping means and said second container is con nected to the suction side of said pumping means; said control means being adapted to shift said changeover valve from said first position to said second position when said boundary surface in said first container reaches said predetermined raised level, and from said second position to said first position when said boundary surface in said second container reaches said predetermined raised level; a piston-cylinder device interposed between said liq uid containers and said changeover valve, said piston-cylinder device separating said passage means into a first section containing a first operating fluid adapted to be pumped by said pumping means and a second section containing a second operating fluid, said boundary surface in each container being formed between said second operating fluid and the slurry, said piston-cylinder device isolating the first operating fluid from the slurry but transmitting hydraulic displacement between said first operating fluid and the slurry, said piston-cylinder device being comprised of first and third cylinders each including a piston, said respective pistons operatively connected together, and second and fourth cylinders each including a piston, said pistons being operatively connected together, said pistons of each cylinder dividing said cylinder into two regions, said first and second cylinders being connected to a first common passage adapted to be selectively connected to the pressure and suction sides of said pumping means by said changeover valve, said first and second cylinders being connected to a second common passage adapted to be selectively connected to the suction and pressure sides of said pumping means by said changeover valve, said first and second common passages respectively communicating with different regions in said first and second cylinders, said third and fourth cylinders being connected to said first and second liquid containers in different regions thereof, said first operating fluid occupying both regions of said first and second cylinders, both regions of said third and fourth cylinders being occupied by said second operating fluid. 4. The improved liquid-piston type slurry pumping system as set forth in claim 3, wherein said first operat- 

1. A liquid-piston type slurry pumping system operable in a slurry pipe line, said system comprising: a pump unit of a type continuously delivering a pressurized operating fluid which is of a specific gravity different from that of slurry to be pumped and is substantially immiscible with, insoluble in, and unreactive with the slurry; at least two valve boxes each including a suction valve and a discharge valve connected respectively to the pipe line; at least a pair of liquid containers connected respectively to the valve boxes; passage means connecting said containers and said pump unit; means for changing over said passages between the pump unit and the liquid containers, thereby to introduce the operating fluid into the liquid containers alternately; detecting means provided in each of the containers for detecting any displacement in excess of a predetermined value of a boundary surface formed between the fluid and the slurry coexisting in the containers; a controller for controlling the operation of the fluid-passage changeover means in response to signals from said detecting means, whereby the pulsations of the boundary surfaces in the containers are controlled in response to the detected results of the detecting means, and the operation of the pumping system is thereby stabilized; and a piston-cylinder device provided between the liquid containers and said fluid-passage changeover means, said device being comprised of means isolating the operating fluid from the slurry but transmitting hydraulic displacement between the operating fluid and the slurry, first and third cylinders respectively including pistons connected together operatively, and second and fourth cylinders respectively including pistons connected together operatively, the first side of the piston in the first cylinder and the second side of the piston in the second cylinder being connected to a passage in said fluid passages changing over means, the second side of the piston in the first cylinder and the first side of the piston in the second cylinder being connected to another passage in said fluid passages changing over means, the first side of the piston in the third cylinder and the second side of the piston in the fourth cylinder being connected to one liquid container, the second side of the piston in the third cylinder and the first side of the piston in the fourth cylinder being connected to the other liquid container, said operating fluid occupying both sides of each of the first and second cylinders, and the interiors of the third and fourth cylinders being occupied by an additional operating fluid which forms a boundary surface in each of the liquid containers between itself and the slurry.
 2. a liquid-piston type slurry pumping system as set forth in claim 1 wherein said operating fluid used in the pumping unit is a gas such as air or steam.
 3. In a liquid-piston type slurry pumping system for conducting a slurry through a pipeline and including pumping means having a suction side and a pressure side and continuously delivering a pressurized operating fluid having a specific gravity lower than that of the slurry and substantially immiscible with, insoluble in, and unreactive with the slurry, at least one pair of valve boxes connected, respectively, to the pipeline in parallel relationship to each other and each including a suction valve and a discharge valve, at leAst one pair of liquid containers respectively connected to said valve boxes, a boundary surface formed in each of said containers between operating fluid and slurry contained therein, passage means connecting said containers and said pumping means, means for changing over said passage means to alternatively introduce the operating fluid delivered from said pumping means into one of said containers, detector means for detecting a level of said boundary surface in each of said containers, and control means for controlling the operation of said changeover means in response to detection of the level by said detecting means, the improvements comprising: said detector means for each container being comprised of means for detecting a displacement of said boundary surface beyond a predetermined raised level in the container, each container being connected at an upper portion thereof to said passage means; said changover means being comprised of a single sliding changeover valve movable between a first position in which a first of said containers is connected to the suction side of said pumping means and a second container is connected to the pressure side and a second position in which said first container is connected to the pressure side of said pumping means and said second container is connected to the suction side of said pumping means; said control means being adapted to shift said changeover valve from said first position to said second position when said boundary surface in said first container reaches said predetermined raised level, and from said second position to said first position when said boundary surface in said second container reaches said predetermined raised level; a piston-cylinder device interposed between said liquid containers and said changeover valve, said piston-cylinder device separating said passage means into a first section containing a first operating fluid adapted to be pumped by said pumping means and a second section containing a second operating fluid, said boundary surface in each container being formed between said second operating fluid and the slurry, said piston-cylinder device isolating the first operating fluid from the slurry but transmitting hydraulic displacement between said first operating fluid and the slurry, said piston-cylinder device being comprised of first and third cylinders each including a piston, said respective pistons operatively connected together, and second and fourth cylinders each including a piston, said pistons being operatively connected together, said pistons of each cylinder dividing said cylinder into two regions, said first and second cylinders being connected to a first common passage adapted to be selectively connected to the pressure and suction sides of said pumping means by said changeover valve, said first and second cylinders being connected to a second common passage adapted to be selectively connected to the suction and pressure sides of said pumping means by said changeover valve, said first and second common passages respectively communicating with different regions in said first and second cylinders, said third and fourth cylinders being connected to said first and second liquid containers in different regions thereof, said first operating fluid occupying both regions of said first and second cylinders, both regions of said third and fourth cylinders being occupied by said second operating fluid.
 4. The improved liquid-piston type slurry pumping system as set forth in claim 3, wherein said first operating fluid is a gas such as air or steam. 